There are currently no reliable, clinically available, non-invasive means to estimate respiratory resistance (R) during inspiration in mechanically ventilated patients who have spontaneous respiratory efforts. Calculation of resistance requires knowledge of the force applied to the respiratory system which, in such patients, includes a component related to pressure generated by respiratory muscles (Pmus). This component continuously changes during the inflation phase and cannot be estimated without prior knowledge of respiratory mechanics. Furthermore, to isolate the component of total applied pressure that is dissipated against resistance (Pres), it is necessary to subtract the pressure used against the elastic recoil of the respiratory system. This requires knowledge of passive respiratory elastance (E) which is also difficult to determine in the presence of unquantifiable Pmus. At present, therefore, R can be reliably estimated only by use of esophageal catheters, which add another invasive intervention to already much instrumented patients, or by elimination of respiratory muscle pressure output with paralysis, or hyperventilation (controlled mechanical ventilation, CMV). The latter entails additional personnel time and does not lend itself to frequent determination of R. To the extent that R is a highly dynamic property that may change frequently, due to secretions or changes in bronchomotor tone, availability of continuous estimates of R may be helpful in the clinical management of such patients. Thus, changes in R can be rapidly identified and dealt with. Furthermore, this information makes it possible to adjust the level of assist according to the prevailing R values, a feature that is of particular utility in pressure assisted modalities of ventilatory support (Pressure Support Ventilation, Proportional Assist Ventilation).
In U.S. Pat. No. 5,884,622 (Younes), assigned to the assignee hereof, an approach is described to determine resistance under similar conditions, namely in assisted ventilation. This prior approach consists of applying at least two different types of transient changes in flow in the course of the inflation phase of the ventilator. The changes in airway pressure (Paw), flow ({dot over (V)}), and volume (V) during these transient flow changes are compared with the time course of these variables in unperturbed breaths. While this approach is capable of providing accurate information about R, it has several limitations. First, because of considerable breath-by-breath variability in the time course of Paw, {dot over (V)} and V in spontaneous unperturbed breaths, it is necessary to average large numbers of perturbed and unperturbed breaths in order to arrive at the real change that occurred during the perturbation. Accordingly, information about resistance is delayed until a sufficiently large number of observations has been averaged. Furthermore, for the same reasons, any true change in patient's resistance is not detected in a timely way. Second, this approach requires at least two different kinds of perturbations. Because, as indicated earlier, a large number of observations is required with each perturbation, this requirement delays the acquisition of reliable information further. Third, the need to average large numbers of breaths and a large number of data points from each breath, greatly increases the computing and storage requirements of the computer used to process the information to provide the value of R. This requirement adds further strain on the extensive and highly complex operations carried out by modern, computer controlled ventilators.